This invention relates generally to magnetic resonance imaging (MRI) and, more particularly the invention relates to decreasing magnetic field sensitivity of long RF excitation pulses.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.0, of the precession of the nuclei is the product of the magnetic field B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =Z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defined Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation proportional to the integral of the pulse over time.
Multi-dimensional RF pulses are finding a growing range of applications in MRI. As performance demands increase (either toward smaller volumes or sharper profiles), hardware limitations on peak RF, gradient slew rates, and/or gradient amplitudes necessitate the use of longer pulse durations. However, this makes the pulses more sensitive to off-resonance effects. Various strategies have been employed to minimize pulse times and hence B.sub.0 sensitivity. Designing the gradients to the hardware limits or non-uniformly sampling excitation k-space are two examples.
The present invention is directed to an improved method and apparatus for decreasing magnetic field (B.sub.0) sensitivity.